The vertebrate immune system requires multiple signals to achieve optimal immune activation; see, e.g., Janeway, Cold Spring Harbor Symp. Quant. Biol. 54:1-14 (1989); Paul William. E., ed. Raven Press, N.Y., Fundamental Immunology, 4th edition (1998), particularly chapters 12 and 13, pages 411 to 478. Interactions between T lymphocytes (T cells) and antigen presenting cells (APC) are essential to the immune response. Levels of many cohesive molecules found on T cells and APC's increase during an immune response (Springer et al., A. Rev. Immunol. 5:223-252 (1987); Shaw and Shimuzu, Current Opinion in Immunology, Eds. Kindt and Long, 1:92-97 (1988)); and Hemler, Immunology Today 9:109-113 (1988)). Increased levels of these molecules may help explain why activated APC's are more effective at stimulating antigen-specific T cell proliferation than are resting APC's (Kaiuchi et al., J. Immunol. 131:109-114 (1983); Kreiger et al., J. Immunol. 135:2937-2945 (1985); McKenzie, J. Immunol. 141:2907-2911 (1988); and Hawrylowicz and Unanue, J. Immunol. 141:4083-4088 (1988)).
T cell immune response is a complex process that involves cell-cell interactions (Springer et al., A. Rev. Immunol. 5:223-252 (1987)), particularly between T and accessory cells such as APC's, and production of soluble immune mediators (cytokines or lymphokines) (Dinarello (1987) New Engl. Jour. Med. 317:940-945; Sallusto (1997) J. Exp. Med. 179:1109-1118). This response is regulated by several T-cell surface receptors, including the T-cell receptor complex (Weiss (1986) Ann. Rev. Immunol. 4:593-619) and other “accessory” surface molecules (Allison (1994) Curr. Opin. Immunol. 6:414-419; Springer (1987) supra). Many of these accessory molecules are naturally occurring cell surface differentiation (CD) antigens defined by the reactivity of monoclonal antibodies on the surface of cells (McMichael, Ed., Leukocyte Typing III, Oxford Univ. Press, Oxford, N.Y. (1987)).
Early studies suggested that B lymphocyte activation requires two signals (Bretscher (1970) Science 169:1042-1049) and now it is believed that all lymphocytes require two signals for their optimal activation, an antigen specific or clonal signal, as well as a second, antigen non-specific signal. (Janeway, supra). Freeman (1989) J. Immunol. 143:2714-2722) isolated and sequenced a cDNA clone encoding a B cell activation antigen recognized by MAb B7 (Freeman (1987) J. Immunol. 138:3260). COS cells transfected with this cDNA have been shown to stain by both labeled MAb B7 and MAb BB-1 (Clark (1986) Human Immunol. 16:100-113; Yokochi (1981) J. Immunol. 128:823; Freeman et al., (1989) supra; Freeman et al. (1987), supra). In addition, expression of this antigen has been detected on cells of other lineages, such as monocytes (Freeman et al., supra).
T helper cell (Th) antigenic response requires signals provided by APCs. The first signal is initiated by interaction of the T cell receptor complex (Weiss, J. Clin. Invest. 86:1015 (1990)) with antigen presented in the context of class II major histocompatibility complex (MHC) molecules on the APC (Allen, Immunol. Today 8:270 (1987)). This antigen-specific signal is not sufficient to generate a full response, and in the absence of a second signal may actually lead to clonal inactivation or anergy (Schwartz, Science 248:1349 (1990)), The requirement for a second “costimulatory” signal provided by the MHC has been demonstrated in a number of experimental systems (Schwartz, supra; Weaver and Unanue, Immunol. Today 11:49 (1990)). The molecular nature of this second signal is not completely understood, although it is clear in some cases that both soluble molecules such as interleukin (IL)-1 (Weaver and Unanue, supra) and membrane receptors involved in intercellular adhesion (Springer, Nature 346:425 (1990)) can provide costimulatory signals.
CD28 antigen, a homodimeric glycoprotein of the immunoglobulin superfamily (Aruffo and Seed, Proc. Natl. Acad. Sci. 84:8573-8577 (1987)), is an accessory molecule found on most mature human T cells (Damle et al., J. Immunol. 131:2296-2300 (1983)). Current evidence suggests that this molecule functions in an alternative T cell activation pathway distinct from that initiated by the T-cell receptor complex (June et al., Mol. Cell. Biol. 7:4472-4481 (1987)). Monoclonal antibodies (MAbs) reactive with CD28 antigen can augment T cell responses initiated by various polyclonal stimuli (reviewed by June et al., supra). These stimulatory effects may result from MAb-induced cytokine production (Thompson et al., Proc, Natl. Acad. Sci. 86:1333-1337 (1989); and Lindsten et al., Science 244:339-343 (1989)) as a consequence of increased mRNA stabilization (Lindsten et al. (1989), supra). Anti-CD28 mAbs can also have inhibitory effects, i.e., they can block autologous mixed lymphocyte reactions (Damle et al., Proc. Natl. Acad. Sci. 78:5096-6001 (1981)) and activation of antigen-specific T cell clones (Lesslauer et al., Eur. J. Immunol. 16:1289-1296 (1986)).
Some studies have indicated that CD28 is a counter-receptor for the B cell activation antigen, BBB-1 (Linsley et al., Proc. Natl. Acad. Sci. USA 87:5031-5035 (1990)). The B7/BB-1 antigen is hereafter referred to as the “B7 antigen”. The B7 ligands are also members of the immunoglobulin superfamily but have, in contrast to CD28, two Ig domains in their extracellular region, an N-terminal variable (V)-like domain followed by a constant (C)-like domain.
Delivery of a non-specific costimulatory signal to the T cell requires at least two homologous B7 family members found on APC's, B7-1 (also called B7, B7.1, or CD80) and B7-2 (also called B7.2 or CD86), both of which can deliver costimulatory signals to T cells via CD28. Costimulation through CD28 promotes T cell activation.
Using genetic fusions of the extracellular portions of B7 antigen and CD28 receptor, and Immunoglobulin (Ig) C.gamma.1 (constant region heavy chains), interactions between CD28 and B7 antigen have been characterized (Linsley et al., J. Exp. Med. 173:721-730 (1991)). Immobilized B7Ig fusion protein, as well as B7 positive CHO cells, have been shown to costimulate T cell proliferation.
T cell stimulation with B7 positive CHO cells also specifically stimulates increased levels of transcripts for IL-2. Additional studies have shown that anti-CD28 MAb inhibited IL-2 production induced in certain T cell leukemia cell lines by cellular interactions with a B cell leukemia line (Kohno et al., Cell. Immunol. 131-1-10 (1990)).
CD28 has a single extracellular variable region (V)-like domain (Aruffo and Seed, supra). A homologous molecule, CTLA-4 has been identified by differential screening of a murine cytolytic-T cell cDNA library (Brunet (1987) Nature 328:267-270).
CTLA-4 is a T cell surface molecule that was originally identified by differential screening of a murine cytolytic T cell cDNA library (Brunet et al., Nature 328:267-270 (1987)). CTLA-4 is also a member of the immunoglobulin (Ig) superfamily; CTLA-4 comprises a single extracellular Ig domain. CTLA-4 transcripts have been found in T cell populations having cytotoxic activity, suggesting that CTLA-4 might function in the cytolytic response (Brunet et al., supra; Brunet et al., Immunol. Rev. 103-21-36 (1988)). Researchers have reported the cloning and mapping of a gene for the human counterpart of CTLA-4 (Dariavach et al., Eur. J. Immunol. 18:1901-1905 (1988)) to the same chromosomal region (2q33-34) as CD28 (Lafage-Pochitaloff et al., Immunogenetics 31:198-201 (1990)). Sequence comparison between this human CTLA-4 DNA and that encoding CD28 proteins reveals significant homology of sequence, with the greatest degree of homology in the juxtamembrane and cytoplasmic regions (Brunet et al., 1988, supra; Dariavach et al., 1988, supra).
Some studies have suggested that CTLA-4 has an analogous function as a secondary costimulator (Linsley et al., J. Exp. Med. 176:1595-1604 (1992); Wu et al., J Exp. Med. 185:1327-1335 (1997) Lindsley, P. et al. U.S. Pat. Nos. 5,977,318; 5,968,510; 5,885,796; and 5,885,579). However, others have reported that CTLA-4 has an opposing role as a dampener of T cell activation (Krummel (1995) J. Exp. Med. 182:459-465); Krummel et al., Int'l Immunol. 8:519-523 (1996); Chambers et al., Immunity. 7:885-895 (1997)). It has been reported that CTLA-4 deficient mice suffer from massive lymphoproliferation (Chambers et al., supra). It has been reported that CTLA-4 blockade augments T cell responses in vitro (Walunas et al., Immunity. 1:405-413 (1994)) and in vivo (Kearney (1995) J. Immunol. 155:1032-1036), exacerbates antitumor immunity (Leach (1996) Science. 271:1734-1736), and enhances an induced autoimmune disease (Luhder (1998) J. Exp. Med. 187:427-432). It has also been reported that CTLA-4 has an alternative or additional impact on the initial character of the T cell immune response (Chambers (1997) Curr. Opin. Immunol. 9:396-404; Bluestone (1997) J. Immunol. 158:1989-1993; Thompson (1997) Immunity 7:445-450). This is consistent with the observation that some autoimmune patients have autoantibodies to CTLA-4. It is possible that CTLA-4 blocking antibodies have a pathogenic role in these patients (Matsui (1999) J. Immunol. 162:4328-4335).
Non-human CTLA-4 antibodies have be used in the various studies discussed above. However, one of the major impediments facing the development of in vivo therapeutic and diagnostic applications for antibodies in humans is the intrinsic immunogenicity of non-human immunoglobulins. For example, when immunocompetent human patients are administered therapeutic doses of rodent monoclonal antibodies, the patients produce antibodies against the rodent immunoglobulin sequences; these human anti-mouse antibodies (HAMA) neutralize the therapeutic antibodies and can cause acute toxicity. These and other deficiencies in the previous antibodies are overcome by the provision of human antibodies to CTLA-4 by the present invention.